Cut zucchini in half lengthwise; scoop out flesh, leaving 1/4-in. shells. Brush with 2 teaspoons oil; set aside. Chop zucchini flesh. , In a large skillet, saute flesh and onion in remaining oil. Add garlic; cook 1 minute longer. Add bread crumbs; cook and stir until golden brown, about 2 minutes. , Remove from the heat. Stir in the mozzarella cheese, mint and salt. Spoon into zucchini shells. Sprinkle with Parmesan cheese. , Grill, covered, over medium heat until zucchini is tender, 8-10 minutes.